1. Field of the Invention
The present invention relates to telecommunication systems and particularly to circuitry for supplying precise tone signals in a telephone central office.
2. Description of the Prior Art
Since the requirement for use of precise tones in telephone central office equipment has existed, the circuitry most commonly used employs two L/C oscillators to produce two precise tone frequencies. These two tones are then mixed and fed to an output amplifier to drive the distribution paths for use throughout the telephone central office. A level adjustment permits the output level and therefore the final tone level at the main distributing frame to be established. Thus by controlling the level of the supply circuit the output is controlled.
A transfer circuit is frequently incorporated into the design so that each of two tone circuits, for any given signal, is fed to about half of the telephone central office load. Upon failure of one of the circuits, the portion of the load that is supplied, is transferred to the other circuit. Circuitry like that described is provided along with telephone central office equipment as manufactured by major telephone communication manufacturers such as GTE Automatic Electric and Western Electric.
The desired approach has several shortcomings. Because the oscillators are of the L/C type, they are known to drift in frequency, with time, temperature and voltage variations.
In the present invention the output of the frequency dividers selected, are summed and fed through a low pass filter to an output amplifier, which includes a certain amount of gain variable over a small range, to compensate for component variation and losses within the telephone central office. If required, failure detectors and transfer circuitry of various types well known in the telephone communications field may be included to operate in a manner similar to previous circuits. The present circuitry typically is used in conjunction with several different attenuator pads, all of which have a 900 OHM output impedance, but which may have different amounts of loss. Through the use of such pads, signals consisting of the same combination of tones but differing in amplitude may be generated by the same cards, rather than having to use a separate set of cards for each different amplitude of each tone combination.
By means of the present invention a universal circuit is provided so that upon installation into the telephone central office equipment the precise tone and correct signal combination will be provided as long as the card is properly strapped or programmed. With the present arrangement only one circuit card will be required for each frequency combination and external interruption will not be required. No frequency drift problem exists.